In the field of semiconductor device fabrication, it is known to perform isotropic lateral etching of a sacrificial layer when manufacturing certain semiconductor devices. For example, a so-called “Silicon-On-Nothing” (SON) Metal Oxide Semiconductor Field Effect Transistor (MOSFET) makes use of a Silicon-Germanium (SiGe) sacrificial layer to disposed a thin silicon layer beneath a gate stack that serves as an active region for the MOSFET, the use of the thin silicon active region providing significant device performance advantages. As part of the fabrication process of the MOSFET, the Si—Ge sacrificial layer is laterally etched and replaced with an oxide buffer layer.
In order to etch the Si—Ge sacrificial layer, either a wet chemical etch solution or an isotropic plasma etching technique is employed in a downstream or remote plasma tool, such as a Chemical Downstream Etcher (CDE) or a Decoupled Plasma Source (DPS)].
Whilst the use of isotropic plasma etching results in a very selective etch due to the electrochemical potential difference between silicon and silicon-germanium, it is difficult to control and cannot be readily monitored in-situ. Further, plasma etching for lateral etches does not currently provide adequate selectivity since the selectivity is lost when the silicon-germanium etch is completed or the etch endpoints is reached. For some applications, an additional masking layer of either resist, or a hard mask of oxide, may also have to be provided to enable plasma etching to be used.
In relation to the use of wet chemical etching solutions, these also suffer from a number of disadvantages. Indeed, whilst also selective, use of such solutions can result in mechanical damage to extremely small and delicate thin features of a device being etched. Additionally, progress of a wet chemical etch is difficult to monitor as no directly measurable parameter is available, for example by means of optical emission. Further, use of wet chemical etching solutions for lateral etching can be limited to the wetting of the etching solution in nano-sized features. Also, like DPS, wet chemical etching is also difficult to control and suffers from a loss of selectivity at Si—Ge endpoints.